Well, have you tried another server besides ntp.ubuntu.com? It may be down for you or too far geographically so it disregards that as suitable. Try pool.ntp.org or <country-tld>.pool.ntp.org (e.g. nl.pool.ntp.org). And do you have a working internet connection accepting outbound NTP traffic?
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gertvdijkJul 22 '13 at 8:53